Twenty years later: updating the status of the osprey Pandion haliaetus in the Cabo Verde Islands, West Africa

Abstract

The first countrywide census of the osprey Pandion haliaetus breeding population in the Cabo Verde Islands took place from December 1997 to September 2001. Since then, about 20 years have elapsed without a new full assessment of the species situation in the country. Yet, some studies and monitoring work have been done in various islands, especially from 2016 onwards. Most notably, long-term monitoring has been carried out on the island of Boavista, recently followed by similar annual surveys on Sal. Although not covering the whole archipelago adequately, data currently available are sufficiently robust for allowing a reliable update of the species situation throughout most of its range and evincing its evolution since the turn of the century. The number of osprey pairs increased remarkably across most of the northern islands, which contain the large majority of the breeding population, while there are apparent signs of some recovery in the southern islands. This growth has presumably resulted from the gradual abandonment of the human harvesting of eggs and nestlings, a formerly widespread practice up to the mid-20th century that has now become rare. The osprey breeding population in Cabo Verde is currently estimated at about 115 pairs, showing an increase of over 30% in the last 20 years.

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Zoologia Caboverdiana 8, 1, 03–10
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Artigo original | Original article
Twenty years later: updating the status of the osprey
Pandion haliaetus in the Cabo Verde Islands, West Africa
Luís Palma 1, *, Samir Martins 2, Rosiane Fortes 3, Isabel Rodrigues 4,
Marcos Hernández-Montero 5 & Rui Freitas 3
1 CIBIO/InBIO, Centro de Investigação em Biodiversidade e Recursos Genéticos, Laboratório Associado,
Universidade do Porto, Campus Agrário de Vairão, 4485-661 Vairão, Portugal
2 BIOS.CV, Conservação do Ambiente e Desenvolvimento Sustentável, Sal Rei, Boavista, Cabo Verde
3 Faculdade de Engenharia e Ciências do Mar, Universidade de Cabo Verde, CP 163, Mindelo, São
Vicente, Cabo Verde
4 Biosfera I, Mindelo, São Vicente, Cabo Verde
5 Projecto Biodiversidade, Mercado Municipal, Santa Maria, Sal, Cabo Verde
*Corresponding author e-mail: luis.palma@cibio.up.pt
RESUMO
O primeiro censo global da população reprodutora de guincho Pandion haliaetus nas ilhas de
Cabo Verde realizou-se entre Dezembro de 1997 e Setembro de 2001. Desde então, passaram
cerca de 20 anos sem uma nova avaliação da situação geral da espécie no arquipélago. No
entanto, decorreram alguns estudos e trabalho de monitorização em várias ilhas, especialmente
após 2016. É de realçar a monitorização de longo termo conduzida na ilha da Boavista,
recentemente seguida por semelhantes levantamentos anuais na ilha do Sal. Apesar de nem todo
o arquipélago ter sido convenientemente coberto, os dados actualmente disponíveis são
suficientemente sólidos para uma actualização fidedigna da situação da espécie ao longo da
maior parte da distribuição da mesma e para evidenciar a evolução desta desde o início do
século. O número de casais de guincho aumentou de forma notável na maioria das ilhas
setentrionais, que albergam a grande maioria da população reprodutora, enquanto nas ilhas
meridionais há indícios de alguma recuperação. Este crescimento resultou provavelmente do
gradual abandono da colheita de ovos e crias nidícolas pelos humanos, uma prática comum até
meados do século XX, que agora se tornou rara. A população reprodutora de guincho em Cabo
Verde está actualmente estimada em cerca de 115 casais, mostrando um aumento de mais de
30% nos últimos 20 anos.
Palavras-chave: Arquipélago, Atlântico Norte, ave de rapina, monitorização, situação actual

Palma et al. 4 Osprey status update in Cabo Verde
ABSTRACT
The first countrywide census of the osprey Pandion haliaetus breeding population in the Cabo
Verde Islands took place from December 1997 to September 2001. Since then, about 20 years
have elapsed without a new full assessment of the species situation in the country. Yet, some
studies and monitoring work have been done in several islands, especially from 2016 onwards.
Most notably, long-term monitoring has been carried out on the island of Boavista, recently
followed by similar annual surveys on Sal. Although not covering the whole archipelago
adequately, data currently available are sufficiently robust for allowing a reliable update of the
species situation throughout most of its range and evincing its evolution since the turn of the
century. The number of osprey pairs increased remarkably across most of the northern islands,
which contain the large majority of the breeding population, while there are apparent signs of
some recovery in the southern islands. This growth has presumably resulted from the gradual
abandonment of the human harvesting of eggs and nestlings, a formerly widespread practice up
to the mid-20th century that has now became rare. The osprey breeding population in Cabo
Verde is currently estimated at about 115 pairs, showing an increase of over 30% in the last 20
years.
Keywords: Archipelago, North Atlantic, raptor, present status, monitoring
INTRODUCTION
The Palearctic osprey, Pandion haliaetus
haliaetus (Linnaeus, 1758) is widely
distributed across Eurasia and the
Mediterranean with projections into the
southern edge of the Palearctic in Africa
(sensu Roselaar 2006), in Cabo Verde and the
Red Sea (Palma et al. 2004, Monti et al. 2015,
Habib 2019). Ospreys are considered resident
in Cabo Verde (Monti et al. 2018a) and show
genetic affinity with northern European
ospreys, so probably originated from birds
wintering in West Africa that settled in the
archipelago (Monti et al. 2018b).
The archipelago of Cabo Verde comprises
10 islands and several islets of volcanic origin
located in the Atlantic about 570 km off West
Africa. Ospreys in Cabo Verde nest from
coastal sand dunes to mountain peaks far
inland (Naurois 1987, Palma et al. 2004), and
live off variable fish diets according to the
island (Martins et al. 2011) and the exposition
to trade winds (Fortes 2016). Apparently,
ospreys have always been widespread in the
north of the archipelago, previously known to
harbour circa 94% of the population
(Palma et al. 2004) and much scarcer in the
south, presumably due to different prey
availability driven by ecological factors
(Naurois 1987). Higher exposition to the
northeasterly trade winds that blow during
most of the year (Duarte & Romeiras 2009)
and to the Canary current is associated with
higher prey availability and richer osprey
diets. This fact provides a possible
explanation for the species higher abundance
in northward and eastward facing coasts at
island scale, and its higher abundance in the
northern islands (Martins et al. 2011, Fortes
2016).
Up to the mid-20th century, Cabo Verde
ospreys were strongly affected by the
overharvest of eggs and nestlings by humans
(Naurois 1969, 1984). This practice decreased
thereafter, allowing numbers to grow from
46–71 in the 1960s (Naurois 1987) to 72–81
pairs towards the end of the century at the
time of the first countrywide assessment,
carried out in 1998 and 1999 with a
complementary survey in Santo Antão in 2001
(Palma et al. 2004).
After 1999, ospreys were surveyed in
Boavista in 2001–2002, with an estimate of

Palma et al. 5 Osprey status update in Cabo Verde
10–14 pairs (López-Suárez 2012), and in
2004–2007 when the number of breeding
pairs grew from 14 to 17 (Siverio et al. 2013).
A partial shoreline survey was also
undertaken in 2003 (Ontiveros 2003). After a
gap in 2008–2011, annual monitoring
resumed in 2012 by the NGO BIOS.CV. On
Sal, after an opportunistic survey in 2016 by
L. Palma, monitoring has been carried out
yearly since 2017 by the NGO ‘Projecto
Biodiversidade’. On São Nicolau, a
comprehensive assessment was done in
January-April 2016 (Fortes 2016). Sectors
uncheckable by land were visited by boat, and
only 6% of the coastline remained
unsurveyed. Old records and osprey-related
place names (see Palma 2017 for
contextualisation) were also checked.
The objective of this work was updating
the osprey population estimate in the country
about 20 years past the 1998–2001 census.
MATERIAL AND METHODS
To update the osprey status in the country we
reviewed all available post-2000 information,
including publications (Ontiveros 2003,
Siverio et al. 2013), theses (Fortes 2016) and
the unpublished yearly monitoring reports of
BIOS.CV and ‘Projecto Biodiversidade’.
Otherwise, surveys were undertaken by the
authors in islands lacking recent information
with the purpose of checking the species
current distribution and numbers, as described
below.
Among the northern islands, São Vicente
was surveyed in March 2016, except for the
southeast coast (circa 20% of the coastline)
due to its inaccessibility from land. Raso and
Santa Luzia were surveyed in April 2016 and
April 2017, respectively. Branco was visited
in May and September 2017 (K. Delgado,
pers. comm.). In the southern islands, only
fragmentary recent data was available.
Therefore, short visits were made to Maio,
Santiago, Fogo, and Brava in May–June 2017
and to Santiago and Maio in June 2019. On
Maio, old records and information provided
by residents and the Maio Biodiversity
Foundation were also checked. Furthermore,
key informants questioned about recent
osprey sightings on Santiago, Fogo and
Rombos provided useful complementary
information (C. Monteiro, N. Barbosa, A.
Leal, A. Veiga, D. Alinho, pers. comm.).
....The reliability of the data available for
updating the numbers of ospreys pairs on each
island was assessed according with the
following data quality categories:
0= anecdotal/ fragmentary data (no update
possible); 1= incomplete survey (unreliable
update); 2= single comprehensive survey
(reasonable update); 3= repeated
comprehensive surveys (reliable update); and
4= systematic annual surveys (very reliable
update).
We used a stepwise approach to calculate
the percentages of change in the population
between 1998–2001 (Palma et al. 2004) and
2016–2019 (present work). First, we
compared the recent (2016 onwards) estimates
in the areas reliably surveyed with those from
the previous survey within the corresponding
areas. Current numbers for Boavista and Sal
were extracted respectively from López-
Suárez (2019) and Hernández-Montero (2019).
Then, we calculated the present
countrywide total estimate by adding to the
recent cumulative estimate: 1) the number of
pairs identified during the previous census
that presumably persist within the areas now
left unsurveyed, i.e. Santo Antão and minor
parts of São Vicente and São Nicolau (see
above), assuming stability as the
precautionary guess (within the general
upward trend); and 2) the numbers suspected
on the islands of the Sotavento. Finally, we
compared the current total estimate with the
previous total to calculate the percentage of
change in the archipelago.

Palma et al. 6 Osprey status update in Cabo Verde
RESULTS
All northern islands with the exception of
Santo Antão were well monitored, so we
consider the respective estimates reliable. The
updating of the osprey population in Santo
Antão was hindered by the lack of human and
logistic resources to tackle the large size and
abrupt relief of the island. In contrast with the
northern islands in general, we considered the
numbers presumed for most of the south
unreliable (Fig. 1), although Fogo, Brava and
Maio were better covered than Santiago.
Fig. 1. Current osprey population estimates and their update reliability in the Cabo Verde Archipelago.
The sizes of the circles are proportional to the estimated number or mean number of pairs (see Material
and Methods for details). Branco and Raso are represented by a common cumulative circle. Numbers in
brackets indicate uncertainty due to incomplete surveys. The darker the grey shading of the islands the
higher the update reliability.
Although variably, the numbers of
breeding pairs increased on most of the
northern islands, from 28 to 37% on average,
and only Santa Luzia and Raso showed
stability or very slight population growth. On
all well-surveyed islands, trends were stable
or positive (Table 1). Overall, the increase
reached on average 35.5% in the area
surveyed and 32.5% in the whole archipelago.

Palma et al. 7 Osprey status update in Cabo Verde
Table 1. Present osprey population estimates (number of pairs) and trends over the last 20 years. (*) Only
six in the area now surveyed; (**) percentages of change were calculated in relation to the area surveyed
and corresponding ends of the interval; (***) see Material and Methods for further explanations. Numbers
in brackets are uncertain.
Island group Island 1998–2001 Present % change**
Northern Santo Antão 18–23 unavailable unknown
São Vicente 8* 13–18 54–67
Santa Luzia 5–6 5–6 0
Branco 1–2 2–3 33–50
Raso 4–5 4–6 0–17
São Nicolau 17 18–22 6–23
Sal 4 11 64
Southern Boavista 11 18 39
Maio 1 (2–3) unreliable
Santiago 3–4 (3–4) unreliable
Fogo 0 (2–3) unreliable
Brava 0 (0) unreliable
Rombos 0 1 0–100
All In area surveyed 48–51 71–84 32–39
Total*** 72–81 103–125 30–35
DISCUSSION
We showed that the osprey breeding
population in Cabo Verde grew by about one
third during the last two decades, reaching the
present estimate of 103–125 pairs
(considering that there was no increase in
Santo Antão and other unsurveyed areas in the
northern islands). Even though it was not
possible to update the population of Santo
Antão, we assumed that its trend echoes the
situation in the relatively similar nearby island
of São Nicolau, i.e., the population remained
roughly stable or slightly increased due to the
natural protection afforded to nest sites by the
rugged relief.
The southern islands remained poorly
covered due to time and logistic constraints,
especially Santiago because of its larger size
and mountainous landscape. Surveying
Santiago requires considerable time and
resources, aggravated by the fact that some
ospreys may be nesting far from the sea.
There is at least one record from 2008 of a
nest site 3.5 km inland (Cesarini & Furtado
2016). Although further fieldwork is needed
in the south of the archipelago, there are signs
of some recovery from the precarious
situation during the 1960s. For instance, an
osprey nest was observed on Ilhéu de Cima in
June 2018 (N. Barbosa & A. Leal, pers.
comm.), the first on the Rombos Islets since
1965 (Naurois 1987).
The stronger increases are found on the
highly populated São Vicente, as well as on
Sal and Boavista with more accessible terrain
(making nest sites vulnerable). However,
monitoring on Boavista showed inter-annual
fluctuation (17.6 ± 2.1; 14–21 pairs), probably
depending on the ecological conditions in the
early breeding season, ultimately determining
how many pairs attempt to breed, and thereby
can be counted. For example, choosing the 21
pairs of 2016 instead of the 18 pairs of 2019
(López-Suárez 2019) would raise the
population increase rate by 9% (from 39 to

Palma et al. 8 Osprey status update in Cabo Verde
48%). Therefore, calculating trends depends
on the year considered.
Interestingly, the only relatively large
northern island with no apparent change is
Santa Luzia, the only one that long remained
uninhabited. So, we think that the upward
trends observed on most of the northern
islands do reflect the recovery of the species
in the region. To this adds the signs of
recovery in the south. Altogether, there is
strong support for assuming a robust
countrywide recovery from the depleted
osprey population of the mid-20th century,
already evident at the turn of the century
(Palma et al. 2004). Looking back to the
estimates of the 1960s on some smaller
islands and islets that were probably well
surveyed by Naurois (1987), current recovery
seems approaching pre-depletion numbers.
For example, Naurois found only 1–2 pairs on
Raso, no pairs on Rombos, and only two pairs
on Santa Luzia (Fig. 1) stating that 10–20 years
before all 5–6 nests or groups of nests he
observed there were occupied (Naurois 1969).
We think that the update deficiencies did
not affect our countrywide assessment of the
osprey breeding population because present
numbers on Santo Antão are not expected to
be much different from 2001; while in the
south numbers are too low to be influential.
Still, a thorough appraisal of the situation in
the south is of utmost importance, because it
probably represents the species demographic
periphery, possible a population sink, likely to
reflect the overall situation in the archipelago
(Dias 1996). The reoccupation of the area by
recruits from the north would probably
confirm the overall regrowth, whereas the
abandonment of nest sites would probably
mean decline due to depleted recruitment.
CONCLUDING REMARKS
It is important to stress that apart from the
now rare nest raiding, there are no other direct
human behaviours negatively affecting the
osprey in Cabo Verde as we could realise
from our own observations since the end of
the last century. Persecution of adult birds in
particular, may have been fairly uncommon in
the past and is extremely rare nowadays, as
shown by the great tameness of the birds. To
this probably contributed the absence of
shotgun hunting activity in the country. The
situation sharply contrasts with the long
history of systematic persecution in Western
Europe that drove the species to extinction in
many countries during the 20th century,
although there is now a general and steady
comeback to the continent (Palma 2001,
Schmidt-Rothmund et al. 2014). The exception
is on the Canary Islands where the species
continues to decline (Siverio et al. 2018).
On the opposite sense goes the growing
impact of disturbance from tourism-related
activities upon coastal nest sites on Boavista
and Sal that jointly hold almost 30% of Cabo
Verde ospreys (Palma et al. 2004, Siverio et
al. 2013). That often results in breeding failure
and is gradually causing the withdrawal of
nest sites away from the coastline,
undoubtedly a major concern for the species
conservation (Palma et al. 2004, Siverio et al.
2013). Still, the positive population trend of
the osprey in the country represents a good
opportunity to implement preventive
conservation actions, above all focusing on
the control and reversal of impacts from
unregulated tourism-related activities.

Palma et al. 9 Osprey status update in Cabo Verde
ACKNOWLEDGEMENTS
We are indebted to K. Delgado, C. Monteiro,
A. Veiga, D. Alinho, N. Barbosa, A. Leal &
T. Militão for the field data provided and to
A. Vieira, A. Lopes, J. Graça, E. Neves, K.
Lopes, A. L. Tavares, and A. Borges for
helping on surveys. We are grateful to
‘Biosfera I’ and Maio Biodiversity
Foundation, delegations of the Ministry of
Agriculture and the Environment in Maio,
São Nicolau and Sal, and Fogo Natural Park
for the logistic support. We thank Cabeólica
S.A. and BIOS.CV for using data from
unpublished monitoring reports from Boavista,
as to ‘Projecto Biodiversidade’ alike in
relation to Sal. We also thank Cabeólica S.A.
and The International Osprey Foundation for
funding osprey studies and monitoring.
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